For example, in IEEE802.11 of a wireless LAN standard, it is defined that bit interleaving is performed on a bit sequence obtained by mapping to a complex plane according to a convolution coding and a modulation method in order to increase error resistance.
The bit interleaving includes inter-symbol interleaving for spreading a burst error in a viterbi decoding on a receiving side and intra-symbol interleaving for spreading robustness in one symbol.
FIG. 1 illustrates an overview of inter-symbol interleaving. In the inter-symbol interleaving, processing is performed in which a bit sequence is written longitudinally in a storage area of a memory by a predetermined number of bits (in the case of FIG. 1, 16 bits), and the written bit sequence is read out transversely by a predetermined number of bits (in the case of FIG. 1, NCBPS/16 bits, NCBPS is the number of a coding bit per one symbol).
In the intra-symbol interleaving, processing is performed in which the bit sequence read out transversely from the memory is shifted in the symbol per one symbol so as to distribute strong and weak positions in robustness of each bit. The intra-symbol interleaving is specifically described.
FIG. 2 illustrates a signal space diagram of a 64QAM modulation method (hereinafter, referred to simply as 64QAM) employed in IEEE802.11ac.
64QAM is a uniform constellation in which 64 signal points are disposed so that the distances between signal points on a complex plane become equal. As for 64QAM, one symbol representing a position of the signal point includes 6 bits.
FIG. 3 illustrates each robustness of 6 bits constituting one symbol of 64QAM. In the case of 64QAM, the robustness of each bit has three phases. In specific, the 0th bit (MSB) and the third bit from a left side of the 6 bits are the strongest, the first bit and the fourth bit from the left side of the 6 bits are the second strongest, and the second bit and the fifth bit (LSB) from the left side of the 6 bits are the weakest.
FIG. 4 illustrates an overview of intra-symbol interleaving corresponding to 64QAM. In the case of QAM, a bit sequence of S bit is shifted per S bit (in the case of 64QAM, 3 bits) of half of the number of bits constituting one symbol according to the row of the memory from which the bit sequence has been read out.
In specific, the bit sequence corresponding to one symbol read out from the 0th row is shifted to the left side by 0 digit per 3 bits (that is, it is not shifted). The bit sequence corresponding to one symbol read out from the first row is shifted to the left side by 1 digit per 3 bits. The bit sequence corresponding to one symbol read out from the second row is shifted to the left side by 2 digits per 3 bits. The bit sequence corresponding to one symbol read out from the third row is shifted to the left side by 3 digits per 3 bits (that is, it is not shifted similar to that in the 0th row). The bit sequence corresponding to one symbol read out from the fourth row is shifted to the left side by 4 digits per 3 bits (that is, it is shifted to the left side by 1 digit per 3 bits, similar to that in the first row). In this manner, in the intra-symbol interleaving corresponding to 64QAM, one symbol is divided per 3 bits, and is shifted in a three-row cycle.
It is assumed that for the above-described intra-symbol interleaving, the employed modulation method is the uniform constellation such as 64QAM.
Incidentally, in these days, in order to perform more efficient wireless communication, the modulation method such as 64NUC (non-uniform constellation) has been proposed in which signal points are disposed at positions where the distances between signal points on a complex plane are not equal (for example, see PTL 1).